The present invention concerns the improved process for preparing diazinon which, owing to its good insecticidal and acaricidal properties, is of great commercial value for the destruction of insect pests.
Diazinon was produced for the first time by Glysin and Margot by reacting 2-isopropyl-4-methyl-6-hydroxypyrimidine (hereafter hydroxypyrimidine) with diethylthiophosphoryl chloride (hereinafter TPC) in an inert solvent, in the presence of potassium carbonate as described in United Kingdom Pat. No. 713,278.
In this heterogeneous reaction the potassium pyrimidinolate is initially formed by heating the hydroxypyrimidine with potassium carbonate in benzene, with simultaneous removal of the water formed. The potassium salt so produced is then reacted with TPC by heating for several hours, the potassium chloride formed extracted by washing with water, and the solvent removed under reduced pressure.
The standard process for the industrial manufacture of diazinon is carried out essentially by means of a 4-stage synthesis, wherein the above-mentioned heterogeneous reaction is the last stage. This last stage is usually carried out using a base such as sodium hydroxide in place of potassium carbonate.
Since 1952 several processes have been described in the literature, which modify this fourth(last) stage. Thus, U.S. Pat. No. 4,066,642 describes simultaneously reacting TPC and the hydroxypyrimidine in the presence of an acid acceptor such as sodium or potassium hydroxide while refluxing the inert solvent to remove the water as it it is formed without the use of any catalyst.
U.S. Pat. No. 4,326,059 describes the preparation of water-free sodium pyrimidinolate by refluxing sodium hydroxide with the hydroxypyrimidine in an aromatic solvent such as benzene in the presence of a phase transfer catalyst to remove any water by azetropic distillation and reacting the thus formed slurry with TPC.
Various other processes involving the use of a variety of catalysts to shorten the reaction times have been described. Suitable catalysts disclosed are mercury salts (U.S. Pat. No. 3,107,245), copper chloride (U.S. Pat. No. 3,107,246), copper nitrate (U.S. Pat. No. 3,367,935), and basic copper oxide (Japanese patent specification number 7,524,958).
In the processes which involve the use of catalysts, it was found that significant amounts of the highly toxic by-products, such as thiotepp (mono- or dithionotetraethylpyrophosphate), are formed. The presence of even small amounts of these by-products in diazinon is undesireable, from the point of view of operators or warm blooded animals that may come into contact therewith. Thus, U.S. Pat. No. 3,432,503 discloses a method of removing these by-products by refluxing the diazinon in an inert solvent in the presence of a base such as sodium hydroxide. However, this process not only requires a separate step subsequent to manufacture but also involves considerable loss of product during workup.
Recently U.S. Pat. No. 4,323,678 reported a one-pot reaction wherein .beta.-isobutyrylaminocrotonic acid amide is cyclized in the presence of an alcohol, the resulting sodium pyrimidinolate precipitated by addition of a non-polar solvent, the alcohol/water removed by fractionation, and the sodium pyrimidinolate reacted with TPC at a temperature of 100.degree. to 130.degree. C.
However all of the above methods suffer from a variety of drawbacks. Examples are long reaction times, the need to use catalysts, the need for tricky manipulation of solvents, and/or the need for a separate step to remove toxic by-products. And none of these processes affords high yields of very pure diazinon.